Involvement of u‐PA in the anti‐apoptotic activity of TGFβ for vascular smooth muscle cells

Previous studies suggest a role for the plasminogen or fibrinolytic system in the activation of latent‐transforming growth β (L‐TGFβ) into active TGFβ. In the present study, the anti‐apoptotic activity of TGFβ on cultured vascular smooth muscle cells (SMC) isolated from the aorta of transgenic mice with single inactivation of genes encoding the tissue‐type plasminogen activator (t‐PA −/− ), urokinase‐type plasminogen activator (u‐PA −/− ), urokinase receptor (u‐PAR −/− ) or plasminogen (Plg −/− ) genes was examined. Latent‐TGFβ inhibited serum deprivation‐induced apoptosis of SMC isolated from wild‐type and t‐PA −/− mice but failed to reduce apoptosis of SMC isolated from u‐PA −/− , u‐PAR −/− or Plg −/− mice. Active TGFβ, however, was able to inhibit serum deprivation‐induced apoptosis of these 5 cell types, indicating that u‐PA and/or plasmin were involved in the activation of L‐TGFβ. The anti‐apoptotic effect of L‐TGFβ could not be evoked by addition of exogenous t‐PA to u‐PA −/− cells, but was revealed by addition of exogenous u‐PA or plasmin. This effect was dependent on the catalytic activity of plasmin as revealed by the dose‐dependent inhibition of aprotinin or epsilon aminocaproic acid (EACA). These results therefore indicate that, at least in vitro, u‐PA‐mediated plasmin, through the generation of active TGFβ from L‐TGFβ, is required for the anti‐apoptotic activity of TGFβ on SMC.